The present embodiments relate to an operating method for a magnetic resonance system.
Operating methods for magnetic resonance systems are described in, for example, the paper “Iterative RF Pulse Design for Multidimensional, Small-Tip-Angle Selective Excitation” by Chun-yu Yip et al., Magnetic Resonance in Medicine, Volume 54 (2005), pages 908 to 917. The paper “Velocity-Selective RF Pulses in MRI” by Ludovic de Rochefort et al., Magnetic Resonance in Medicine, Volume 55 (2006), pages 171 to 176, also describes an operating method.
The excitation and later detection of magnetic resonance signals may be performed with the aim of examining the tissue of an examination object (e.g., a human being). The morphological information may be relevant. Site-selective excitation of the magnetic resonance signals may take place. In the case of examinations of this kind, variations in the Larmor frequency of the relevant type of nucleus caused by inhomogeneities of the basic magnetic field and/or by chemical displacement may result in artifacts on reconstruction.
It is known from the above-named paper by Chun-yu Yip et al. how to determine high-frequency transmit pulses and gradient currents correlated therewith. Using the high-frequency transmit pulses and the gradient currents, in the case of site-selective excitation, excitation may also take place when variations in the Larmor frequency of the relevant type of nucleus may not be excluded.
For this, Chun-yu Yip et al. utilize the approach
                              M          ⁡                      (                          r              →                        )                          =                  ⅈ          ⁢                                          ⁢          γ          ⁢                                          ⁢                                    M              0                        ⁡                          (                              r                →                            )                                ⁢                                    ∫              0              T                        ⁢                                          HF                ⁡                                  (                  t                  )                                            ⁢                              exp                (                                  ⅈ                  ⁢                                                                          ⁢                                                            k                      →                                        ⁡                                          (                      t                      )                                                        ⁢                                      r                    →                                                  ⁢                                                                  )                            ⁢                              ⅆ                t                                                                        (        1        )            Here, M is the magnetization actually occasioned at a specific site {right arrow over (r)}. γ is the gyromagnetic ratio, and M0 is basic magnetization. HF is a basic amplitude curve (e.g., an envelope curve) as a function of time. {right arrow over (k)} is a factor determined in the usual way from the gradient magnetic field at the site in question.
Chun-yu Yip et al. inserted an additional term into the above equation allowing for the phase development due to spatial variation Δω ({right arrow over (r)}) of the Larmor frequency. The modified equation is as follows
                              M          ⁡                      (                          r              →                        )                          =                  ⅈ          ⁢                                          ⁢          γ          ⁢                                          ⁢                                    M              0                        ⁡                          (                              r                →                            )                                ⁢                                    ∫              0              T                        ⁢                                          HF                ⁡                                  (                  t                  )                                            ⁢                              exp                [                                                      ⅈ                    ⁢                                                                                  ⁢                                                                  k                        →                                            ⁡                                              (                        t                        )                                                              ⁢                                          r                      →                                                        ⁢                                                                          +                                      ⅈ                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                                          ω                      ⁡                                              (                                                  r                          →                                                )                                                              ⁢                                          (                                              T                        -                        t                                            )                                                                      ]                            ⁢                              ⅆ                t                                                                        (        2        )            
Unlike equation 1, equation 2 may not be solved analytically. However, equation 2 may be solved using iterative solving methods such that the resultant magnetization M corresponds to a prespecified target magnetization or that a standard representing a measure for the difference between actual magnetization and target magnetization is minimized.
Chun-yu Yip et al. mention that k-space based selective excitation has been used in a range of magnetic resonance applications. Mentioned as examples are functional artifact correction, blood velocity measurement, parallel excitation using an array of transmit coils and excitation inhomogeneity correction.
It is known from the above-named paper by Ludovic de Rochefort et al. how to determine high-frequency transmit pulses and gradient current correlating therewith. Using the high-frequency transmit pulses and the correlating gradient current, velocity-selective excitation of atomic nuclei may take place. However, the transmit pulses recommended by Ludovic de Rochefort et al. are sensitive to variations in the Larmor frequency (e.g., the transmit pulses) are not independent.